Advances with charged particle beam transfer devices as used for making integrated circuits and related devices have made it possible to achieve both improved transfer pattern resolution and improved throughput (productivity) in recent years. One such device that has been investigated in the past uses a one-shot transfer system that transfers one die (meaning an entire pattern formed on a single layer for one of multiple integrated circuits) or multiple dies' worth of patterns from a mask onto a sensitive substrate in a single exposure ("shot"). However, with one-shot transfer systems, the masks which comprise the transfer original are difficult to make, and it is difficult to keep aberrations in the charge particle optical system (hereinafter referred to simply as the "optical system") to below a desired value inside of a large optical field of one die or greater. Therefore, devices using divided transfer systems have recently been studied, which divide the pattern, or total image field, which is to be transferred onto the substrate into multiple subfields, each subfield being smaller than the size of one die. In this divided transfer method, each subfield can be transferred and individually corrected for optical aberrations like focus, displacement, and distortion of the transferred image, so that good resolution and precise positioning of the image can be obtained over a wider total image field than with the one-shot transfer method.
However, sufficiently precise methods for the correction of image positioning errors, such as focal point errors and distortions of the projected image, have not been developed for the divided transfer method of image projection. In a previous attempt to measure and correct image positioning errors, first, a pattern forming a subfield of a mask is transferred onto a wafer. As is typical, the pattern is reduced in size by the projection optical system prior to being transferred on the wafer. Optical aberrations of a projected image are calculated according to the distance from the mask pattern subfield to the charged particle beam axis. The projected image is corrected as a function of this distance.
As the required density of integrated circuits on a die has increased, the required feature size of transferred images has decreased. When a large image is created by stitching together several projected images on the wafer, achieving the necessary precision is even more difficult. Such precision requirements demand precise placement of a subfield pattern onto the wafer, including precise reduction factors and rotation angles of the projected image. If these requirements are not satisfied, the subfields are not connected smoothly and the stitched-together large image lacks the required designed precision.
To maintain pattern formation characteristics, such as reduction factors and rotation angles of a projected mask subfield, within a set range, the actual image forming characteristics, such as reduction factor and rotation angle, must be measured precisely.
Although methods for the measurement of actual projected images have been previously disclosed, such methods are only suitable for correction of very small projected images.
Specifically, previously disclosed methods for the measurement of projected images are suitable for projected images of about (5 .mu.m).sup.2 in area, as shown for example in Japanese laid-open patent H 7-22349. In such methods, several images are projected onto marks capable of reflecting or scattering charged particle beams, and the reflected or scattered charge particle beam are detected by a detector disposed near the marks. When a deflector scans an image formed by a charged particle beam over the marks, a signal is generated having several peaks due to the varying intensity of the scattered or reflected charge particles as the marks are scanned.
Using such charged particle beam mark detection methods, each subfield can be transferred with focus and distortion corrections, so that good resolution and precise positioning of the image can be obtained over a wider area than with the one-shot transfer method. A reduction factor and a rotation angle of the projected images are calculated utilizing the distance values between the detected signals. However, these measurement methods using charged particle beam deflection tend to have low precision due to the instability of deflection sensitivity of charged particle beams, such as electron particle beams, and due to varying rotational error of the deflection axis (causing scanning direction errors).
Nonetheless, using such a measurement method is sufficiently precise when the projected image is small, i.e., about (5 .mu.m).sup.2, and when the required accuracy of the reduction factor is not too high. However, when using the divided transfer method, a subfield area on a mask is, typically, in the order of about 1 mm.sup.2. If the reduction factor is one quarter the size of the mask pattern, as is typical, then the projected image of the subfield pattern will be (250 .mu.m).sup.2. Consequently, known methods do not provide methods and apparatus capable of obtaining sufficiently precise measurements of the projected images. Accordingly, methods and apparatus are needed in order to measure and correct pattern formation characteristics, so as to produce transferred images having the required performance suitable for wide field range pattern microlithography methods, such as divided transfer methods.